1. Field of the Invention
The present invention relates to an NTSC/PAL signal discriminating circuit and an active filter using such a circuit, and more particularly to a device adapted for automatically adjusting the trap center frequency of an active filter and trap frequency in accordance with the frequency of an input signal.
2. Description of the Prior Art
It is generally known that a variety of circuits have been contrived for automatically adjusting the trap center frequency f.sub.o of an IC active filter employed in an electronic apparatus such as a television receiver or a video tape recorder. A typical conventional circuit for achieving automatic adjustment of the trap center frequency is so formed that a reference signal is fed to a filter or an integrator or the like constituting the filter, and a phase detection or a level comparison is executed with regard to the output of the filter and the reference signal to obtain an error signal. The voltage of such error signal is fed back to automatically adjust the trap center frequency f.sub.o of the active filter. An exemplary configuration of such automatic frequency adjusting circuit is shown in FIG. 1.
In this diagram, an input signal S.sub.in fed to an input terminal T.sub.in is supplied to both a reference filter 1 and a phase detector 2. The signal S.sub.1 produced by shifting the phase of the input signal S.sub.in by 90.degree. in the reference filter 1 is supplied to the phase detector 2, where the difference in phases of such two signals S.sub.in and S.sub.1 is detected. The voltage on a charge/discharge capacitor C, i.e., the voltage at a point A in FIG. 1, is controlled in accordance with the phase detection output thus produced. The point-A voltage is applied to an inverting input terminal of a comparator 3. Meanwhile, a reference voltage V.sub.ref is applied to a non-inverting input terminal of the comparator 3, where the point-A voltage is compared with the reference voltage V.sub.ref to produce an error detection signal S.sub.2. The error detection signal S.sub.2 thus obtained from the comparator 3 is then fed back to the reference filter 1 to thereby control the operation of the reference filter 1 in such a manner that the point-A potential is corrected to a predetermined value, i.e., the error is reduced to zero.
The error detection signal S.sub.2 is supplied to an active filter 4 which is to be adjusted, so that the center frequency f.sub.o of the active filter 4 is automatically adjusted.
A chrominance subcarrier signal f.sub.sc can be used as the aforementioned reference signal. Since the frequency of such subcarrier signal f.sub.sc is different depending on the NTSC system or the PAL system, it is clear that the trap center frequency f.sub.o of the active filter 4 to be adjusted is also different in each mode. Therefore, in an exemplary case where the reference filter 1 is such that the phase is shifted by 90.degree. at a frequency of 3.58 MHz, the center frequency f.sub.o of the active filter 4 is adjusted at 4 MHz for example. In this state, when a subcarrier signal f.sub.sc of the PAL format is input to the circuit of FIG. 1, the reference filter 1 is balanced at a point with a 90.degree. phase shift at 4.43 MHz, so that the trap center frequency of the active filter 4 to be adjusted is deviated from the desired value as follows: EQU f.sub.Trap (PAL)=4 MHz.times.4.43/3.58=4.95 MHz (1)
For preventing such a disadvantage, it is necessary to add or subtract the offset component, which is derived from the difference between the NTSC and PAL formats, to or from the signal for controlling either the reference filter 1 or the active filter 4. And consequently it has been necessary heretofore to input an external switching signal for executing the above action.